1. Field of the Invention
This invention relates to an improvement in electron or ion ("emissive") guns and more specifically relates to suppressing undesirable arcs in such guns by providing a resistance and an inductance in series with the gun electrode leads.
2. Description of the Prior Art
Electron and ion guns are well known in various fields for instance as a source of electrons in electron beam lithography as used for fabrication of semiconductor masks and also in scanning electron microscopes, electron beam testing of integrated circuits and line width metrology measuring equipment as used in the semiconductor industry. Such guns also have application outside of the semiconductor industry, for instance in scanning electron microscopes, generally.
It is a well known deficiency of such electron guns that spurious field emissions from metallic protrusions or by electron-stimulated desorption of gas molecules cause undesirable electrical discharges, i.e. arcs. These arcs damage the emissive properties of the electron guns either due to variation of the work function, i.e., the quantity of energy needed to emit an electron by the gun or by damaging the emitter structure from which the electron currents are emitted.
One example of an electron gun is shown in copending U.S. patent application Ser. No. 07/671,425, Mark A. Gesley, "Low Aberration Field Emission Electron Gun", filed May 21, 1991.
One example of an arc suppressor in the prior art is an attachment to an electron (or ion) gun sold by FEI Co., Beaverton, OR. This arc suppressor includes a conventional single-hole ferrite toroid upon which are wound all of the leads that connect the power supply to the electron gun electrodes. Capacitors are connected between each of the leads. These leads include for example those to the filament, extractor, focus, and suppressor electrodes. The winding scheme is such that all the leads are wound together around the toroid. This configuration has the significant disadvantage that it does not fully suppress the damaging arcs and for certain electrode geometries can actually increase the energy dissipated by the arc when compared to having no arc suppressor at all. A more effective arc suppressor would be highly desirable for extending the life of the cathode electrode of the electron gun.